In concurrently filed patent applications of J. B. Mallos, Ser. No. 140,716 and L. R. Kasday, Ser. No. 140,715 both of which are hereby incorporated by reference herein, there are disclosed, inter alia, touch sensitive arrangements for determining the position of a finger touch on the surface of a CRT screen. In one embodiment there is disclosed a parallel surface device for overlaying the CRT screen through which light from the CRT screen may pass. When the screen is touched at a point on the device total internal reflection occurs and the light from the screen becomes trapped within the device. The trapped light travels to the edge of the device and is detected by photodetectors mounted along the edge. In another embodiment a flexible membrane is placed over the device. As long as an air gap remains between the top surface of the device and the membrane light from the screen will not become trapped within the device. Touching the membrane causes it to deflect into contact with the top surface of the device, thereby causing total internal reflection to occur. Since the reflection occurs at the instant the raster beam passes under the finger, the timing of its occurrence can be used to determine the position of the touch, as is done in light pen situations.
Problems stem, however, in the application of the above principles to a practical system. Imperfections in the surface of the screen, dirt, grease build-up, and geometrical aberrations all combine to cause some total internal reflection to occur at all times, even when no touch has occurred. The result is that the photodetectors produce appreciable output at all instants during which the raster beam is drawing bright areas on the screen.
These problems are compounded in that for each cycle of the raster the output wave form has a non-constant value resulting in a signal which, for a given point, may be greater in magnitude without a touch than is a subsequent portion of the signal output in the presence of a touch. Thus, it is not practical to simply measure the signal level of the photodiode output in order to detect a touch signal.